In the past, an electrolyte in which a film having micro-pores called a separator was impregnated with a non-aqueous electrolytic solution was generally used. A lithium ion secondary battery called a polymer battery employing a polymer electrolyte made of a polymer has recently attracted more attention than such electrolyte employing liquid.
This polymer battery uses an electrolyte made in the form of gel in which the polymer is impregnated with a liquid electrolytic solution. Since it holds a liquid electrolytic solution in the polymer, it has the advantages that there is little possibility of leakage of the liquid and, therefore, safety of the battery is improved and that it has more freedom in adopting the configuration of the battery.
Since lithium ion conductivity of such polymer electrolyte is lower than an electrolyte containing only an electrolytic solution, there has occurred a practice to reduce thickness of the polymer electrolyte. There, however, has arisen a problem in such polymer electrolyte whose thickness is reduced that, since its mechanical strength is reduced, the polymer electrolyte tends to be broken or give rise to a hole during production of the battery resulting in short-circuiting between the positive electrode and the negative electrode.
It has, therefore, been proposed, as disclosed by Japanese Patent Application Laid-open Publication No. 6-140052, to provide a solid electrolyte by adding an inorganic oxide such as alumina to the electrolyte and thereby increase its mechanical strength. As such inorganic oxide, inorganic oxides other than alumina such as silica and lithium aluminate have also been proposed.
However, the addition of such inorganic oxides such as alumina to an electrolyte causes the problem that lithium ion conductivity in the solid electrolyte is significantly reduced. Moreover, when charging and discharging are repeated in a lithium ion secondary battery employing this solid electrolyte, the electrolyte reacts with such inorganic oxide resulting in deterioration in the charging-discharging characteristic of the lithium ion secondary battery.
Furthermore, the lithium ion secondary battery comprising a polymer electrolyte cannot avoid thermal expansion and contraction and abrupt change in ion conductivity caused by change in temperature due to characteristics of the polymer used in the electrolyte. Therefore, the lithium ion secondary battery comprising such polymer solid electrolyte has poor heat resistance and cannot be used over a broad temperature range.
It is, therefore, an object of the present invention to provide an all solid state lithium ion secondary battery which has high heat resistance and can be used over a broad temperature range, has a high battery capacity and an excellent charging-discharging characteristic, and can be used stably for a long period of time.
It is another object of the invention to provide a solid electrolyte used in this lithium ion secondary battery.